Never underestimate the value of a customer testimonial or reference letter. Whether you sell a product or service on the Internet or by direct sales force (or both), these are gold!
Prospective customers feel more comfortable making a "buy" decision if your business has glowing endorsements from existing customers. In their eyes, this reduces the risk of selecting your company as their vendor. So, always ask for a reference letter or testimonial upon completing projects or transactions.
If your business has performed as agreed, there will be no problem getting this endorsement. You can actually ensure this by writing it yourself and then emailing the "draft" to the customer to get their blessing, asking them "if they want to make any changes."
Place one-to-two paragraph testimonials in conspicuous places on your Website. If you have a direct sales force, make sure customer reference letters are printed on the customer's letterhead and signed. Often, a prospect will even forego calling references if your sales people give them high-caliber endorsement letters at an appropriate stage in the sales cycle. These are great closing tools!